<html>
    <head></head>
    <body>
        <div>
            <table width=300;>
                <tr>
                <th style="border: 1px solid black; position: relative;">

                    <p style="margin:0px" onclick="showDiv()">ddddd
                    </p>
                    
                        <div id="div1" style="width: 100%; min-height: 50px; background-color:blue; position: absolute; display: none">
                            <select style="height: 30px">
                                <option>1</option>
                                <option>2</option>
                            </select>
                            <button onclick="hiddenDiv()" >确定</button>
                        </div>
                    </div>
                </th>
                <th style="border: 1px solid black">2</th>
                <th style="border: 1px solid black">3</th>
            </tr>
            </table>
        </div>
    </body>
</html>

<script>
    function showDiv() {
        document.getElementById("div1").style.display = "block"
    }

    function hiddenDiv() {
        document.getElementById("div1").style.display = "none"
    }
</script>